The objective of facial expression recognition (FER) is to predict basic expressions such as happy, disgust, neutral, sad, Surprised, and others, from facial images or videos [1]}, [2]}. Applications of such systems include emotion-aware multimedia and smart devices [3]}, personal mood management [4]}, [5]}, and others. FER is challenging due to a number of reasons including variations in how different people express different emotions, different lighting or background conditions, and more. Moreover, the intensities with which emotions are expressed throughout the face can vary among subjects or even for the same subject in different contexts.
For these reason, there has been an increased effort toward facial expression recognition in recent years from both images [1]}, [2]} and videos [8]}, [2]}, [10]}.
